Cattermole’s 19th century painting commemorates the Second Diet of Speyer, which occurred on this day in 1529. The next day, the Diet’s Lutheran members lodged an official appeal against Archduke Ferdinand‘s suppression of evangelical theology, delivering the titular Letter of Protest that gave the nascent religious movement its present name—Protestant.

Herbet Minton Cundall’s A History of British Watercolour Painting (New York, 1908) notes that Cattermole was offered a knighthood after completing this work, but the painter declined the honour.